It is known that preformed articles of pulverulent materials can be highly densified or post-densified by isostatic hot-pressing to produce substantially pore-free shaped polycrystalline articles the density of which may reach 100% of the theoretical density.
If the pulverulent materials per se or articles preformed therefrom having open pores, that is to say, having pores open at the surface, are subjected to an isostatic hot-pressing operation, it is necessary to provide them with a gas-tight casing to prevent the gas used as pressure-transfer medium from penetrating and interfering with densification.
The so-called glass-capsule technique was developed for that purpose. According to that technique, for example, pulverulent materials per se are placed in prefabricated glass capsules or glass containers the internal dimensions of which correspond approximately to the desired shape of the finished shaped article, the choice of the glass being dependent on the temperature required for densifying the powders. The glass containers filled with the powder are evacuated and sealed gas-tight. The sealed containers, preheated to deformation point to reduce shock, are introduced into a hot molten-salt bath and subsequently heated under pressure to the required temperature at which the powder densifies and the container becomes soft (see DEPS 19 01 766, which corresponds to U.S. Reissue Pat. No. 28,301).
Preformed articles having open pores also can be placed in prefabricated glass capsules that do not have to be exactly adapted to the dimensions of the article (see DE-AS No. 23 46 499, which corresponds to British Patent No. 1,441,330, and DE-OS No. 25 48 740, which corresponds to British Patent No. 1,522,705), it being possible additionally to fill the space between the glass capsule and the preformed article with a glass powder that has a higher softening temperature than that of the capsule itself (see DE-AS No. 26 01 294, which corresponds to British Pat. No. 1,529,966). Before being introduced into the isostatic hot press, the capsules and their contents are evacuated and then sealed gastight. During the heating-up process in the high pressure autocalve, the glass capsules and the optionally present glass powder soften and, due to the acting gas pressure, conform to the dimensions of the preformed article.
Instead of being introduced into prefabricated glass capsules, preformed articles can be coated directly with a glass or vitreous paste, for example, by flame-spraying or plasma-spraying or by dipping, to form a porous surface layer, which is then fused by heating under reduced pressure to form a gas-tight casing (see DE-AS No. 22 08 250, which corresponds to British Pat. No. 1,374,033). In the case of preformed articles that require a high densification temperature, the casing produced must consist of a high-melting vitreous paste to prevent the casing from running off prematurely or penetrating into the article. This has the disadvantage, however, that the surface layer becomes gas-tight only at a fairly high temperature.
A process has been developed for preformed articles that require a high densification temperature, for example, those consisting of silicon nitride, in which two porous layers are applied to the preformed article, an inner layer being applied directly to the article, consisting of a high-melting glass-forming material and the outer layer consisting of a lower-melting glass-forming material. First, the outer layer is melted gas-tight by heating under reduced pressure, and then pressure is applied with increasing temperature so that, before reaching and while maintaining the required densification temperature, the article is always enclosed by a gas-tight casing (see DE-OS No. 27 02 073, which corresponds to U.S. Pat. No. 4,112,143).
Since, however, such glass encapsulation techniques obviously do not bring about a tight enclosure of the preformed article, methods have been devised in which the escape of gases from the pulverulent article during the formation of the casing is prevented by controlling the gas pressure during the conversion of the gas-permeable casing into a casing that is not permeable to the pressure medium (see DE-OS No. 29 15 831, which corresponds to British Patent No. 2,024,256A, and DE-OS No. 29 16 223, which corresponds to British Pat No. 2,024,255A).
In all cases where the so-called glass capsule technique is used, irrespective of whether prefabricated glass capsules are used or the glass casing is produced on the article, there is nevertheless always the danger that the glass casing will run off prematurely. The flow of the glass casing cannot be eliminated by known measures.
For materials that require high densification temperatures, that is to say, temperatures greater than 700.degree. C., the types of glass having the highest softening temperatures such as silica glass, cannot be used as casing materials under certain conditions. In the manufacture of very small shaped articles, the glass casing can be prevented from completely running off at high temperatures for a relatively long time by the supportive effect of the glass that has already run off. In that case, the shaped article is coated at its upper end with only a thin skin of glass whereas the lower end is embedded in a thick layer of glass which prevents uniform densification. In the case of large shaped articles, on the other hand, the glass casing runs off almost completely.